Non-metallic surfaces are usually metallized by first making the respective surface catalytically receptive to electroless metal deposition and subsequently exposing the thus catalyzed surface to a plating bath solution of the kind operating without an external source of electricity and for a time sufficient for forming a metal, e.g., Cu or Ni layer of desired thickness. This initial layer is usually provided with additional metal deposits formed by conventional electroplating. In well known methods for making plated through hole printed circuit boards, this metallizing concept and its variations are employed for metallizing the hole walls. In the version starting with two-sided copper clad laminate as the base material, a panel of suitable size is first provided with the required holes, and rendered catalytically receptive by immersion in a known catalyst solution. Subsequently a metal-, usually copper, deposit is formed by exposure to a bath solution which produces metal deposits without an external source of electricity, generally known as electroless plating baths, and for a time sufficient for achieving a thickness of, e.g., 0.5 to 2.5 microns. This initial, conductive metal-layer is further plated up by means of conventional electroplating.
The typical catalyst solutions employed in the aforedescribed methods have been used in this industry for many years and have been developed to a relatively high degree of stability. Surfaces treated with such solutions catalytically promote the generation of electroless metal deposits by the oxydation of suitable components present in the electroless plating bath with this mechanism acting as an internal source of electrons to be used in the plating reactor by reducing complexed metal ions to metal.
Operation of electroless plating solutions require rather careful monitoring of the different components and replenishing of used up materials by controlled addition of chemicals. Furthermore, the said plating solutions have a tendency to indiscriminate deposition thus forming metal, e.g., copper deposits on walls and the bottom of tanks used for operating such plating baths. This necessitates frequent interruption of the plating operation, removal of the plating solution from the tank and cleaning the tank walls and bottom by means of an etching operation.
Electroless metal plating is, therefore, rather expensive and complex and needs highly trained operators.
In spite of these substantial shortcomings, electroless deposition of an initial layer of metal has, up to now, been an integral part of all processes used for metallizing non-metallic surfaces including such processes employed in the manufacture of printed circuit boards.
Radovsky et al, U.S. Pat. No. 3,099,608, has described the use of palladium-tin-chloride colloid to form an essentially non-conductive film of colloidal or semi-colloidal particles on the hole walls made in a laminate used to make printed circuits; and of electroplating for copperizing said hole walls.
The process of Radovsky et al has, however, severe shortcomings and was found to be not applicable for practical use. The palladium-tin-chloride colloidal suspension has an unacceptably short life span. It can only be used for about nine days due to coagulation of the suspension and, because of its high palladium content is rather expensive. Furthermore, the Radovsky et al method deposits substantially more copper on the surface than on the walls of through holes and is, therefore, unacceptable for commercial use.
Radovsky et al is based on the use of a "thin, barely visible film of particles" of "semi-colloidal palladium" deposited on the surface to be plated, said film having "substantial resistance" and on the teaching "that the palladium being by nature both a catalytic metal and a conductive metal has potentialities for simultaneous and combined activating and conductive functions" (column 4, lines 53 to 56) and further, that "After the electroplating is started at a conductor it is activated apparently by the catalytic properties of the palladium and the electrodeposition process proceeds directly on the film of conductivator particles" (column 4, lines 62 to 66). At column 5, lines 2 to 7 Radovsky et al state: "Since the colloidal palladium deposit in the through holes was an extremely poor conductor to serve as a base for the electroplating as compared with the deposited graphite something else must have aided in the electrodeposition, i.e., a catalyst must have aided in the plating reaction". In spite of the fact that Radovsky et al's observation dates from 1959 and consequently is contemporary with the use of graphite for metallizing non-conductors and with the first application of the "seeder-electroless plating-technology" for metallizing plastic parts and making plated-through-hole (PTH) boards, it did not result in a process of any practical use. Considering the substantial initial difficulties with the seeder-electroless technology and its development, and further, the continuing complexity characteristic of electroless plating bath operation, control and maintenance, when compared to the comparatively simple electroplating process, it is rather most surprising that Radovsky's observations were of no impact as far as the technological development of the last two decades is concerned. The reason is, that Radovsky's observations did not result in a teaching which allows the average person skilled in the art to make use of it. Lacking this teaching, Radovsky's observation could only be duplicated when using his "conductivator-solution" and the copper pyrophosphate electroplating bath existing at the time. It is believed that Radovsky did not recognize the importance of the composition of the copper electroplating bath. For example, of the known formulations for pyrophosphate electroplating bath employed at the time of Radovsky's filing, the simplest one did not produce copper of adequate quality for printed circuit boards; the more complex type of bath did produce adequate copper quality, but inhibits the working of Radovsky's suggested process. The industry, therefore, found Radovsky's observations to be of no practical use. The "seeding-electroless copper, followed or not followed by the electroplating was consequently accepted as the only approach metallizing non-metallic surfaces available to the art.
Radovsky therefore teaches away from the present invention claimed by applicant. To arrive at this invention the misconception presented by Radovsky that the characteristics of the electroplating baths were not critical had to be overcome and completely discarded.